A technique is known in which a file storage migrates a file, written by a host through a communication network, to a cloud storage, and restores the file from the cloud storage in response to reading by the host.
A WAN (Wide Area Network) acceleration technology is used between communication networks, because file transmission through the communication network might take time. The WAN acceleration technology improves the throughput and the response performance of WAN communications. For example, a technique of shortening a response time to the host, when the file storage and the cloud storage are coupled with each other through a CIFS (Common Internet File System), is described in NPL 1. This is achieved by the file storage reading a file from the cloud storage beforehand.